WASHINGTON DC - U.S. President Donald Trump wished good luck to his former national security adviser Michael Flynn on Tuesday, hours ahead of Flynn's sentencing for lying to investigators about his contacts with Russia in the weeks before Trump assumed power in early 2017.

'Will be interesting to see what he has to say, despite tremendous pressure being put on him, about Russian Collusion in our great and, obviously, highly successful political campaign,' Trump said on Twitter. 'There was no Collusion!.'

Special counsel Robert Mueller has recommended that the 60-year-old Flynn, a retired Army general and once head of the country's Defense Intelligence Agency, not be sentenced to any prison time because of his 'substantial' cooperation with prosecutors, although it will be up to U.S. District Judge Emmet Sullivan to decide his sentence.

Flynn served as Trump's national security adviser at the outset of his administration but was fired after less than a month on the job. Evidence surfaced that Flynn had lied to Vice President Mike Pence and others about his contacts with Russia's then-ambassador to Washington, Sergey Kislyak, in the period between Trump's election in November 2016 and becoming president on January 20, 2017.

Flynn was a fixture at Trump's political rallies during the 2016 campaign, often mocking his Democratic challenger, former U.S. Secretary of State Hillary Clinton. At the Republican national convention that formally nominated Trump as the party's 2016 candidate, Flynn led thousands of Trump supporters in chanting against Clinton, 'Lock her up! Lock her up!' for her usage of a private email server during her tenure as the country's top diplomat.

Flynn met extensively with Mueller's investigators since pleading guilty a year ago, talks that drew the interest of Trump just ahead of the sentencing as to what he might reveal about the inner workings of the 2016 campaign.

Flynn becomes the first Trump White House aide to be sentenced in Mueller's wide-ranging 19-month investigation of Trump campaign links to Russia and whether Trump, as president, obstructed justice by trying to thwart the probe.

Last week, however, Trump's former attorney, New York lawyer Michael Cohen, was sentenced to three years in prison, in part for his role, at Trump's direction, of making $280,000 in payments shortly before the election to buy the silence of two women who alleged they had affairs with the future president a decade before he entered politics. Others in Trump's orbit are awaiting sentencing for various offenses.

Flynn had drawn investigators' scrutiny before he ran afoul of the FBI investigation in January 2017. While serving as an adviser to the Trump campaign in 2016, he lobbied for a Dutch company linked to the Turkish government without registering as a foreign agent.

On Monday, the U.S. charged two of Flynn's former business associates with illegally carrying out a Turkish lobbying campaign in the U.S. The government said the scheme was an effort to push American officials to expel a Muslim cleric, Fethullah Gulen, alleged by Turkish President Recep Tayyip Erdogan as orchestrating the failed 2016 coup against Erdogan's government.

Flynn began cooperating with the special counsel after pleading guilty to lying to federal agents about his conversations with Kislyak. He became the first of five former Trump associates who have entered guilty pleas with the special counsel's office.

Trump has for months derided Mueller's probe, tweeting several more broadsides against it after wishing Flynn well.

'The whole Russian Witch Hunt is a Fraud and a Hoax which should be ended immediately,' Trump said.